/*
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF licenses this file
 *  to you under the Apache License, Version 2.0 (the
 *  "License"); you may not use this file except in compliance
 *  with the License.  You may obtain a copy of the License at
 *  
 *    http://www.apache.org/licenses/LICENSE-2.0
 *  
 *  Unless required by applicable law or agreed to in writing,
 *  software distributed under the License is distributed on an
 *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 *  KIND, either express or implied.  See the License for the
 *  specific language governing permissions and limitations
 *  under the License. 
 *  
 */
package org.apache.mina.common;

import java.net.SocketAddress;

import org.apache.mina.filter.ReferenceCountingIoFilter;

/**
 * A filter which intercepts {@link IoHandler} events like Servlet
 * filters.  Filters can be used for these purposes:
 * <ul>
 *   <li>Event logging,</li>
 *   <li>Performance measurement,</li>
 *   <li>Authorization,</li>
 *   <li>Overload control,</li>
 *   <li>Message transformation (e.g. encryption and decryption, ...),</li>
 *   <li>and many more.</li>
 * </ul>
 * <p>
 * <strong>Please NEVER implement your filters to wrap
 * {@link IoSession}s.</strong> Users can cache the reference to the
 * session, which might malfunction if any filters are added or removed later.
 * 
 * <h3>The Life Cycle</h3>
 * {@link IoFilter}s are activated only when they are inside {@link IoFilterChain}.
 * <p>
 * When you add an {@link IoFilter} to an {@link IoFilterChain}:
 * <ol>
 *   <li>{@link #init()} is invoked by {@link ReferenceCountingIoFilter} if
 *       the filter is added at the first time.</li>
 *   <li>{@link #onPreAdd(IoFilterChain, String, NextFilter)} is invoked to notify
 *       that the filter will be added to the chain.</li>
 *   <li>The filter is added to the chain, and all events and I/O requests
 *       pass through the filter from now.</li>
 *   <li>{@link #onPostAdd(IoFilterChain, String, NextFilter)} is invoked to notify
 *       that the filter is added to the chain.</li>
 *   <li>The filter is removed from the chain if {@link #onPostAdd(IoFilterChain, String, org.apache.mina.common.IoFilter.NextFilter)}
 *       threw an exception.  {@link #destroy()} is also invoked by
 *       {@link ReferenceCountingIoFilter} if the filter is the last filter which
 *       was added to {@link IoFilterChain}s.</li>
 * </ol>
 * <p>
 * When you remove an {@link IoFilter} from an {@link IoFilterChain}:
 * <ol>
 *   <li>{@link #onPreRemove(IoFilterChain, String, NextFilter)} is invoked to
 *       notify that the filter will be removed from the chain.</li>
 *   <li>The filter is removed from the chain, and any events and I/O requests
 *       don't pass through the filter from now.</li>
 *   <li>{@link #onPostRemove(IoFilterChain, String, NextFilter)} is invoked to
 *       notify that the filter is removed from the chain.</li>
 *   <li>{@link #destroy()} is invoked by {@link ReferenceCountingIoFilter} if
 *       the removed filter was the last one.</li>
 * </ol>      
 * 
 * @author The Apache Directory Project (mina-dev@directory.apache.org)
 * @version $Rev$, $Date$
 * 
 * @see IoFilterAdapter
 */
public interface IoFilter {
    /**
     * Invoked by {@link ReferenceCountingIoFilter} when this filter
     * is added to a {@link IoFilterChain} at the first time, so you can
     * initialize shared resources.  Please note that this method is never
     * called if you don't wrap a filter with {@link ReferenceCountingIoFilter}.
     */
    void init() throws Exception;

    /**
     * Invoked by {@link ReferenceCountingIoFilter} when this filter
     * is not used by any {@link IoFilterChain} anymore, so you can destroy
     * shared resources.  Please note that this method is never called if
     * you don't wrap a filter with {@link ReferenceCountingIoFilter}.
     */
    void destroy() throws Exception;

    /**
     * Invoked before this filter is added to the specified <tt>parent</tt>.
     * Please note that this method can be invoked more than once if
     * this filter is added to more than one parents.  This method is not
     * invoked before {@link #init()} is invoked. 
     *
     * @param parent the parent who called this method
     * @param name the name assigned to this filter
     * @param nextFilter the {@link NextFilter} for this filter.  You can reuse
     *                   this object until this filter is removed from the chain.
     */
    void onPreAdd(IoFilterChain parent, String name, NextFilter nextFilter)
            throws Exception;

    /**
     * Invoked after this filter is added to the specified <tt>parent</tt>.
     * Please note that this method can be invoked more than once if
     * this filter is added to more than one parents.  This method is not
     * invoked before {@link #init()} is invoked. 
     *
     * @param parent the parent who called this method
     * @param name the name assigned to this filter
     * @param nextFilter the {@link NextFilter} for this filter.  You can reuse
     *                   this object until this filter is removed from the chain.
     */
    void onPostAdd(IoFilterChain parent, String name, NextFilter nextFilter)
            throws Exception;

    /**
     * Invoked before this filter is removed from the specified <tt>parent</tt>.
     * Please note that this method can be invoked more than once if
     * this filter is removed from more than one parents.
     * This method is always invoked before {@link #destroy()} is invoked.
     *  
     * @param parent the parent who called this method
     * @param name the name assigned to this filter
     * @param nextFilter the {@link NextFilter} for this filter.  You can reuse
     *                   this object until this filter is removed from the chain.
     */
    void onPreRemove(IoFilterChain parent, String name, NextFilter nextFilter)
            throws Exception;

    /**
     * Invoked after this filter is removed from the specified <tt>parent</tt>.
     * Please note that this method can be invoked more than once if
     * this filter is removed from more than one parents.
     * This method is always invoked before {@link #destroy()} is invoked.
     *  
     * @param parent the parent who called this method
     * @param name the name assigned to this filter
     * @param nextFilter the {@link NextFilter} for this filter.  You can reuse
     *                   this object until this filter is removed from the chain.
     */
    void onPostRemove(IoFilterChain parent, String name, NextFilter nextFilter)
            throws Exception;

    /**
     * Filters {@link IoHandler#sessionCreated(IoSession)} event.
     */
    void sessionCreated(NextFilter nextFilter, IoSession session)
            throws Exception;

    /**
     * Filters {@link IoHandler#sessionOpened(IoSession)} event.
     */
    void sessionOpened(NextFilter nextFilter, IoSession session)
            throws Exception;

    /**
     * Filters {@link IoHandler#sessionClosed(IoSession)} event.
     */
    void sessionClosed(NextFilter nextFilter, IoSession session)
            throws Exception;

    /**
     * Filters {@link IoHandler#sessionIdle(IoSession,IdleStatus)}
     * event.
     */
    void sessionIdle(NextFilter nextFilter, IoSession session, IdleStatus status)
            throws Exception;

    /**
     * Filters {@link IoHandler#exceptionCaught(IoSession,Throwable)}
     * event.
     */
    void exceptionCaught(NextFilter nextFilter, IoSession session,
            Throwable cause) throws Exception;

    /**
     * Filters {@link IoHandler#messageReceived(IoSession,Object)}
     * event.
     */
    void messageReceived(NextFilter nextFilter, IoSession session,
            Object message) throws Exception;

    /**
     * Filters {@link IoHandler#messageSent(IoSession,Object)}
     * event.
     */
    void messageSent(NextFilter nextFilter, IoSession session, Object message)
            throws Exception;

    /**
     * Filters {@link IoSession#close()} method invocation.
     */
    void filterClose(NextFilter nextFilter, IoSession session) throws Exception;

    /**
     * Filters {@link IoSession#write(Object)} method invocation.
     */
    void filterWrite(NextFilter nextFilter, IoSession session,
            WriteRequest writeRequest) throws Exception;

    /**
     * Represents the next {@link IoFilter} in {@link IoFilterChain}.
     */
    public interface NextFilter {
        /**
         * Forwards <tt>sessionCreated</tt> event to next filter.
         */
        void sessionCreated(IoSession session);

        /**
         * Forwards <tt>sessionOpened</tt> event to next filter.
         */
        void sessionOpened(IoSession session);

        /**
         * Forwards <tt>sessionClosed</tt> event to next filter.
         */
        void sessionClosed(IoSession session);

        /**
         * Forwards <tt>sessionIdle</tt> event to next filter.
         */
        void sessionIdle(IoSession session, IdleStatus status);

        /**
         * Forwards <tt>exceptionCaught</tt> event to next filter.
         */
        void exceptionCaught(IoSession session, Throwable cause);

        /**
         * Forwards <tt>messageReceived</tt> event to next filter.
         */
        void messageReceived(IoSession session, Object message);

        /**
         * Forwards <tt>messageSent</tt> event to next filter.
         */
        void messageSent(IoSession session, Object message);

        /**
         * Forwards <tt>filterWrite</tt> event to next filter.
         */
        void filterWrite(IoSession session, WriteRequest writeRequest);

        /**
         * Forwards <tt>filterClose</tt> event to next filter.
         */
        void filterClose(IoSession session);
    }

    /**
     * Represents write request fired by {@link IoSession#write(Object)}.
     */
    public static class WriteRequest {
        private static final WriteFuture UNUSED_FUTURE = new WriteFuture() {
            public boolean isWritten() {
                return false;
            }

            public void setWritten(boolean written) {
            }

            public IoSession getSession() {
                return null;
            }

            public Object getLock() {
                return this;
            }

            public void join() {
            }

            public boolean join(long timeoutInMillis) {
                return true;
            }

            public boolean isReady() {
                return true;
            }

            public void addListener(IoFutureListener listener) {
                throw new IllegalStateException(
                        "You can't add a listener to a dummy future.");
            }

            public void removeListener(IoFutureListener listener) {
                throw new IllegalStateException(
                        "You can't add a listener to a dummy future.");
            }
        };

        private final Object message;

        private final WriteFuture future;

        private final SocketAddress destination;

        /**
         * Creates a new instance without {@link WriteFuture}.  You'll get
         * an instance of {@link WriteFuture} even if you called this constructor
         * because {@link #getFuture()} will return a bogus future.
         */
        public WriteRequest(Object message) {
            this(message, null, null);
        }

        /**
         * Creates a new instance with {@link WriteFuture}.
         */
        public WriteRequest(Object message, WriteFuture future) {
            this(message, future, null);
        }

        /**
         * Creates a new instance.
         * 
         * @param message a message to write
         * @param future a future that needs to be notified when an operation is finished
         * @param destination the destination of the message.  This property will be
         *                    ignored unless the transport supports it.
         */
        public WriteRequest(Object message, WriteFuture future,
                SocketAddress destination) {
            if (message == null) {
                throw new NullPointerException("message");
            }

            if (future == null) {
                future = UNUSED_FUTURE;
            }

            this.message = message;
            this.future = future;
            this.destination = destination;
        }

        /**
         * Returns {@link WriteFuture} that is associated with this write request.
         */
        public WriteFuture getFuture() {
            return future;
        }

        /**
         * Returns a message object to be written.
         */
        public Object getMessage() {
            return message;
        }

        /**
         * Returne the destination of this write request.
         * 
         * @return <tt>null</tt> for the default destination
         */
        public SocketAddress getDestination() {
            return destination;
        }

        public String toString() {
            return message.toString();
        }
    }
}
